Basically bald. If they use a guard, it's the shortest one possible. All the guys are pretty much bald starting on I-Day, and as soon as it gets long enough to look semi-normal, they all get their heads shaved again.

My DS said, those 5 minutes in the barbers chair was such a relief on I-day; no yelling at you and you could sit down.
We have a great picture of him from WebGuy with his head half shaved with a laughing smile on his face as obviously the barber had said something to him to make him laugh.